Réactions de dissociation des ions de méthane CX4+ (X = H,D) aux faibles énergies: effets isotopiques

Dissociation mass spectra, resulting from the collisions of CX₄⁺ (X = H, D) ions, were studied with a tandem mass spectrometer of longitudinal type. Important isotopic effects were observed in the formation of CX₃⁺ ions. The dependence of these effects on the energy (E_L = 2–25 eV) and the nature of the colliding gas (He, Ne, Ar, Kr, Xe) is presented.     

Etude du désenfumage par extraction mécanique dans un local siège d'un feu: expériences en grandeur et simulations numériques par modèles de zones et de champs

This study regards conditions of control of fire-induced smoke in a 7-m × 7-m rectangular cell, for an ‘intermediate configuration’ of a volume larger than that of an apartment room but smaller than that of an entrance hall, of a fire source a little less powerful than ‘design fires’ considered in standard approaches, of a fan-powered exhaust rate a little higher than rates typically required in regulations.From case to case were experimentally modified the following parameters: ceiling height (3 m, or 6.4 m), source type and power (between 100 and 500 kW), exhaust volume flow rate (from a few tenths of m³.s⁻¹ to a few m³.s⁻¹). Each experiment was supposed to reach a steady-state configuration, which has proved more or less true in practice. Smoke stratification within the cell has proved less sharp for a fire source non located in the centre of the cell, and the mean temperature of gases in the exhaust inlet has proved lower than under the cell ceiling, which suggested a phenomenon of ‘plugholing’ (air mixing upstream). In spite of the weaknesses of the computer models, and of a certain unaccuracy of measurements, plugholing phenomenon could be recognized in simulation results yielded by the zone model (which were in a better accordance with test results for higher exhaust rates), since the field model gave rise to a better agreement between computed and measured results for lower heat release rates and/or higher rates of smoke exhaust.     

The collisional energy transfer between the D 1Π and d 3Π states of the NaK dimer induced by argon buffer gas H. Zhiwei et al.: Appl. Phys. B 66, 471 (1998)

It is shown in this Comment that the method proposed by Zhiwei et al. to extract cross sections for collisional energy transfer between molecular states mediated by argon atoms, is neither correct nor suitable, as it fails to reproduce the molecular behaviour they observe. An alternative model based on a consecutive scheme is put forward, and it is shown that a minimal assumption of one intermediate collisionally activated step is enough to satisfactorily reproduce the observations by Zhiwei et al. However, the extracted parameters are functions of the involved rate constants that are too complex to leave any hope of attaining information on the desired cross sections from them by just varying the argon buffer gas pressure, as done by Zhiwei et al. Received: 30 November 1998 / Published online: 14 July 1999     

Cavity ring down absorption at low temperatures: C–H spectra (Δυ = 1–6) of CH3D and C–H overtones (Δυ = 5, 6) of CH2D2 and CH4

The spectra of the C–H stretch fundamental and overtones (Δυ = 1–6) of CH₃D have been recorded. Absorptions in the visible were obtained with a phase shift cavity ring down technique where an optical cavity is inside a low temperature cryostat. Absorptions below 12,000 cm^?1 were observed with a Fourier transform spectrophotometer. The local mode harmonic frequency and anharmonicity were obtained and used with the harmonically coupled anharmonic oscillator (HCAO) model to calculate energy levels and assign the absorption bands to particular transitions. Overtone absorptions (Δυ = 5 and 6) of CH₄ and CH₂D₂ have also been obtained. The integrated absorption was calculated as a function of the density of the gas samples and used to obtain the band strength and the cross- section of the Δυ = 5 and 6 C–H transitions for each molecule. Cross–sections for CH₄ agree within 10% with traditional absorption measurements with a multiple pass cell at high pressures. The importance of the new experimental technique is emphasized for laboratory studies of planetary atmospheres.